I know i can purchase upgrades, but Yahoo must always be the host? i.e., when and if the company were to get big, and I wanted to go beyond the constraints in my web design, can I do that, or is the site always going to be limited by Yahoo. Basically, can you take it with you when you go?

You own it, you can do anything you want. You can get hosting from some other company or you can even transfer your domain to somewhere else if you dont like yahoo's service.